Design of a gas sensor based on a cladding-reduced long period fiber grating coated with a sensitive film

A gas sensor based on a cladding-reduced long period fiber grating (LPFG) coated with a sensitive film is presented and studied. The sensitivity of this sensor is defined as the relative gradient of transmissivity T to the film refractive index n3 that can be changed by exposing the sensitive film to the measured gas. Based on the vector coupled-mode theory and a model of four-layer fiber structure, the sensing characteristics of this sensor are investigated, with emphasis on the influence of cladding reduction on the sensitivity. By analyzing the dependence of the sensitivity Sn on the cladding radius a2, the film optical parameters (film thickness h3 and refractive index n3) and the grating parameters (grating period Λ, grating length L and core refractive index modulation σ), the optimum combinations of these design parameters are obtained. Compared with the common-cladding LPFG, the sensitivity is significantly improved by reducing the cladding radius; the resolution is enhanced by one order of magnitude and to be 10−10, which provides a theoretical foundation for the design of a high-sensitivity LPFG sensor with reduced cladding for gas sensing.